1. Field of the Invention
The present invention relates to a lighting apparatus provided with a light transmitting-cover or plate for diffusing light rays emitted from a light source.
2. Description of the Prior Art
As well known, there is a lighting apparatus of the type in which the light rays emitted from a light source, such as an incandescent lamp, a low pressure discharge lamp and a high pressure discharge lamp, are diffused through a light transmitting-cover or plate. A lighting apparatus of this type has been used for indoor and outdoor illumination and advertising display purposes. In a lighting apparatus using a combination of the light source and the light transmitting-plate, the light transmitting-plate protects the light source, keeps the light source from sight, and diffuses the light emitted from the light source. In some lighting apparatuses used for the advertising display, the outer surface of the light transmitting-plate is used for the display surface.
In the lighting apparatus, light incident on the light transmitting-plate contains direct light rays directly emitted from the light source, and light rays which are reflected from the inner surface of the lighting apparatus. Thus, direct and reflected light rays are incident upon the light transmitting-plate at various incident angles. The light source, such as an incandescent lamp, a low pressure discharge lamp, and a high pressure discharge lamp, generally have a non-uniformity of luminance and color depending on the direction against light axis. The light emitted from its light emitting section is generally far from uniform in luminance and color. When the light rays emitted from the light source with such characteristics are incident on the light transmitting-plate, the irregularity of illumination and color appears on the light emitting surface of the light transmitting-plate and the surface illuminated by the lighting apparatus of this type.
To solve such problems of the prior lighting apparatus, the light transmitting-plate has been modified variously. For example, a diffusing plate or a refracting plate has been proposed, as the light transmitting-plate, for example, a frosted glass, an opaque plate or a cover with its one surface having a spherical lens or a prism structure.
In the lighting apparatus of the type using the diffusing plate shown in FIG. 1, even if the light source has a poor luminance distribution and a poor uniformity of color, the luminance distribution and color uniformity may be improved on the illuminated surface to some extent, but not completely. The diffusing plates of the lighting apparatus of the type is provided with spherical lens segments on one surface thereof. Light rays A (A-1 to A-5) and light rays B (B-1 to B-5) as indicated by solid and broken lines are incident on a convex surface 1 of the lens segment at different angles and these light rays are emitted from a flat surface 2. In this case, the light rays A (A-1 to A-5) and B (B-1 to B-5) are transmitted through the lens segment, while being refracted as shown. As seen from the figure, the light rays A and B emitted from the flat surface 2 have different center lines in their spread angles. In this situation, if there is an intensity difference or a color difference between the light rays A and B, the intensities or the colors of light rays A' and B' projected from the flat surface 2 are different to each other in areas Pl and P2. In the lighting apparatus using optical refractors with the spherical lens segment elements on one surface, when the light rays from the light source are incident at a certain angle on the refractor, an intensity distribution of the transmitted light rays has its peak in the incident direction, as shown in FIG. 2. Therefore, the light intensity on the illuminated surface is maximized at an area to which the incident light rays are directed. This indicates that there occurs a non-uniformity in the illuminance distribution over the illuminated surface. While the above description relates to the general type of the lighting apparatus, the same thing is true for the display surface for the advertising display. Also in this case, the poor illuminance distribution and poor color balance appear on the light emitting surface of the optical refractor as the display surface. The same problem arises also in the lighting apparatus having a plurality of light sources.